1. Field of the Invention
This invention relates generally to all GMR devices having synthetic, antiparallel coupled pinned layers and, more particularly, to a novel coupling layer that provides greater stability for the coupling.
2. Description of the Related Art
The GMR (giant magneto-resistive) sensor and its operation are easy to describe conceptually, although the technological developments that have led to its present form are quite complex. Basically the GMR sensor consists of two magnetic layers, formed vertically above each other in a parallel planar configuration and separated by a non-magnetic layer. Each magnetic layer has a magnetic moment in the plane of the layer. When a “sense” current passes between the planes it is subject to resistance variations, dR, which are proportional to the angle between the magnetization directions. These variations are measured by voltage variations between the ends of the sensor. The quality of the sensor is measured by what is called the GMR ratio, dR/R, which is the ratio between the maximum resistance variation and the resistance of the sensor.
The evolution of the general GMR configuration, has created a form, called a spin-valve, which includes one layer whose magnetic moment is free to move under the influence of external fields (the “free” layer) and another layer whose magnetic moment is fixed in space (the magnetically “pinned” layer or “reference” layer). These two layers are separated by a non-magnetic layer. This form is presently used in a wide variety of devices and in several basic configurations. In current-in-plane (CIP) configurations, a sense current passes longitudinally along the plane directions of the layers, entering at one lateral edge of the sensor and exiting at the other. In current-perpendicular-to-plane (CPP) configurations, the current passes vertically through the planes, entering at the top and exiting through the bottom. In the tunneling magnetoresistive configuration (TMR), which can be used as a sensor and is now also used in MRAM devices, the free and pinned layers are separated by a dielectric tunneling layer and the angle between the magnetic moments does not directly create a resistance variation but rather determines a quantum mechanical tunneling probability through the barrier layer. In the GMR spin valve read head devices, the free and pinned layers are separated by non-magnetic conducting layers. In all of these devices, however, the pinned layer shares a common role and a common configuration: two layers of ferromagnetic material (called AP1 and AP2) separated by a non-magnetic but electrically conducting coupling (or spacer) layer, with the entire tri-layered structure held magnetically in place by an antiferromagnetic (AFM) pinning layer in the following configuration: AP1/coupling layer/AP2/AFM
More specifically, the antiferromagnetic pinning layer pins one ferromagnetic layer (AP2) in a unidirectional magnetization direction. This unidirectionally pinned layer then couples across the coupling layer with the other ferromagnetic layer (AP1), causing its magnetization to be held in an antiparallel direction to AP2. The process by which the magnetizations are set is an annealing process wherein the configuration is subjected to an external magnetic field at specific temperatures for a certain length of time. Typically, the material composition and thicknesses of AP1 and AP2 are chosen so that the total magnetic moment of the tri-layer is approximately zero. This important method of forming the pinned layer using an approximately zero net magnetic moment tri-layered configuration is disclosed in Heim et al. (U.S. Pat. No. 5,465,185) and later expanded upon in Fontana, Jr., et al. (U.S. Pat. No. 5,701,223), both of which use a Ru layer as the antiferromagnetic coupling layer. Somewhat later Parkin (U.S. Pat. No. 6,153,320) discloses an antiferromagnetically coupled trilayer in which the coupling layer is a ternary alloy of Ru, Os and Re. Gill (U.S. Pat. No. 6,612,018) also discloses a trilayer using a Ru coupling layer, but the ferromagnetic layers coupled thereby are specifically composed of Co90Fe10. Li et al. (U.S. Pat. No. 6,620,530) discloses a trilayer pinned layer in which the coupling layer is a layer of Ru, Ir or Rh which is formed to a thickness of only 4 angstroms. Ordinarily such a thin coupling layer would require very high annealing fields, but the disclosed invention requires lower fields. Finally, Perner et al., (U.S. Pat. No. 6,667,901) discloses a device in which a first magnetic tunnel junction is formed directly on a second magnetic tunnel junction, both junctions including pinned trilayers. As will be discussed below, the present invention teaches a pinned trilayer having performance features that are not found in the prior art cited above.
Because the role of the pinned layer is to retain a spatially fixed set of magnetization directions and to thereby serve as a reference layer for the free layer's moving magnetic moment, it must be coupled in a manner that produces a stable direction of magnetization.
For a GMR sensor with such a pinned layer there are, in fact, three basic requirements:                (1) High dR and dR/R for a high signal.        (2) High Hs, saturation field of the antiparallel coupled pinned layer, for good directional stability.        (3) Correct AP1/AP2 thickness and thickness ratio, for directional stability and a good value of dR/R.        
Although a high Hs is desirable, there is a conflict between too high a value and the limitations of the annealing process required to set the pinned layer field direction. As is well known in the art, a high field magnetic annealing is usually required to establish a well defined pinned direction in the antiparallel pinned structure. During the annealing process a magnetic field is applied at a 150°-350° C. temperature to align the magnetic moments of AP1 and AP2 in a parallel direction. This requires overcoming Hs so that the magnetic moments can be forced into the parallel alignment. Since the maximum available magnetic field in commercially available heating ovens is typically between 10 and 20 kOe, this means that Hs cannot be too great.
It is usually desirable to adjust the thicknesses of the AP1 and AP2 layers for the purpose of optimizing the GMR performance in the CIP or CPP sensor or the TMR performance in a tunneling junction configuration. For example, the dR/R ratio in a CIP configuration reaches a peak value at certain AP1 thicknesses. In the TMR and CPP GMR configurations, dR/R keeps increasing with the AP1 thickness because the spin diffusion length is longer in those configurations. As is known, Hs is a function of exchange coupling and AP1 (AP2) thickness. Hs will decrease with increasing AP1 thickness. To maintain Hs and other stability related properties which are dependent on AP1 thickness and meanwhile meet the requirements set by magnetic annealing capabilities, the coupling strength between the AP1 and AP2 layers should be adjustable within a wide range. The coupling strength, however, depends strongly on the material used for the spacer layer. For example, Ru and Rh are most often used as coupling layer materials. The peak value of the coupling strength obtained with Rh is nearly five times that obtained with Ru. Thus, by using Rh instead of Ru a much stronger coupling between AP1 and AP2 is obtained, but, conversely, magnetic annealing becomes nearly impossible. Thus, for practical purposes, coupling strengths between those provided by Ru and those provided by Rh are needed. For this reason, the present invention proposes a Rh/Ru or Ru/Rh double layer as the coupling layer. With such a double layer a wide range of coupling strengths can be obtained and the advantages of stability and ease of annealing can both be had.